Fuel efficiency
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Fuel efficiency sometimes means the same as thermal efficiency or fuel economy. This is the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. But fuel efficiency can also mean the output one gets for a unit amount of fuel input such as "miles per gallon" for an automobile. Here, vehicle-miles is the output, but for transportation, output can also be measured in terms of passenger-miles or ton-miles (of freight). While the thermal efficiency of petroleum engines has improved in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.
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[edit] Energy-efficiency terminology
"Energy efficiency" is similar to fuel efficiency but the input is usually in units of energy such as British thermal units (BTU), megajoules (MJ), gigajoules (GJ), kilocalories (kcal), or kilowatt-hours (kW·h). The inverse of "energy efficiency" is "energy intensity", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile (of freight transport, for long/short/metric tons), GJ/t (for steel production), BTU/(kW·h) (for electricity generation), or litres/100 km (of vehicle travel). This last term "litres per 100 km" is also a measure of "fuel economy" where the input is measured by the amount of fuel and the output is measured by the distance travelled. For example: Fuel economy in automobiles.
If one knows the heat value of a fuel, it's trivial to convert from fuel units (such as liters of gasoline) to energy units (such as MJ) and conversely. Except that there are two different heat values for the same fuel (see below) and for conversion from electricity to fuel energy, one may need to know how much heat energy from fossil fuel it took to generate the electricity used.
[edit] Energy content of fuel
The specific energy content of a fuel is the heat energy that is obtained by burning a specific quantity of it (like a gallon, litre, kilogram, etc.). It's sometimes called the "heat of combustion". There exists two different values of specific heat energy for the same batch of fuel. One is the high (or gross) heat of combustion and the other is the low (or net) heat of combustion. The high value is obtained when, after the combustion, the water in the "exhaust" is in liquid form. For the low value, the "exhaust" has all the water in vapor form (steam). Since water vapor gives up heat energy when it changes from vapor to liquid, the high value is larger since it includes the latent heat of vaporization of water. The difference between the high and low values is significant, about 8 or 9%. This accounts for most of the apparent discrepancy in the heat value of gasoline. See [1]. In the U.S. (and the table below) the high heat values have traditionally been used, but in many other countries, the low heat values are commonly used.
| Fuel type | MJ/L | MJ/kg | BTU/imp gal | BTU/US gal | Research octane number (RON) |
|---|---|---|---|---|---|
| Regular Gasoline | 31.60 | 42.70 | 151,600 | 126,200 | Min 91 |
| Premium Gasoline | 32.84 | 43.50 | 157,500 | 131,200 | Min 95 |
| Autogas (LPG) (60% Propane + 40% Butane) | 24.85 | 46.02 | 119,200 | 99,300 | 115 |
| Ethanol | 21.17 | 26.80 | 101,600 | 84,600 | 129 |
| Methanol | 15.56 | 19.70 | 74,600 | 62,200 | 123 |
| Gasohol (10% ethanol + 90% gasoline) | 30.63 | 41.11 | 146,900 | 122,300 | 93/94 |
| Diesel | 35.50 | 42.50 | 170,200 | 141,700 | N/A (see cetane) |
[edit] Fuel economy
Fuel economy is usually expressed in one of two ways:
- The amount of fuel used per unit distance; for example, litres per 100 kilometres (L/100 km). In this case, the lower the value, the more economic a vehicle is (the less fuel it needs to travel a certain distance);
- The distance travelled per unit volume of fuel used; for example, kilometres per litre (km/L) or miles per gallon (mpg). In this case, the higher the value, the more economic a vehicle is (the more distance it can travel with a certain volume of fuel).
Converting from mpg or km/L to L/100 km (or vice versa) involves the use of the reciprocal function, which is not distributive. Therefore, the average of two fuel economy numbers gives different values if those units are used. If two people calculate the fuel economy average of two groups of cars with different units, the group with better fuel economy may be one or the other.
The formula for converting miles per US gallon (3.785 L) to L/100 km is 
, where x is the miles per gallon number. For miles per Imperial gallon (4.546 L) the formula is 
.
In Europe, the two standard measuring cycles for "L/100 km" value are motorway travel at 90 km/h and rush hour city traffic. A reasonably modern European supermini may manage motorway travel at 5 L/100 km (47 mpg US) or 6.5 L/100 km in city traffic (36 mpg US), with carbon dioxide emissions of around 140 g/km.
An average North American mid-size car travels 27 mpg (US) (9 L/100 km) highway, 21 mpg (US) (11 L/100 km) city; a full-size SUV usually travels 13 mpg (US) (18 L/100 km) city and 16 mpg (US) (15 L/100 km) highway. Pickup trucks vary considerably; whereas a 4 cylinder-engined light pickup can achieve 28 mpg (8 L/100 km), a V8 full-size pickup with extended cabin only travels 13 mpg (US) (18 L/100 km) city and 15 mpg (US) (15 L/100 km) highway.
An interesting example of fuel economy is the popular microcar Smart Fortwo cdi, which can achieve up to 3.4 L/100 km (69.2 mpg US) using a turbocharged three-cylinder 41 hp (30 kW) Diesel engine. The Fortwo is produced by DaimlerChrysler and is currently only sold by one company in the United States (see external link ZAP). The current record in fuel economy of production cars is held by Volkswagen, with a special production model of the Volkswagen Lupo (the Lupo 3L) that can consume as little as 3 litres per 100 kilometres (78 miles per US gallon or 94 miles per Imperial gallon). The last Lupo was built in July 2005.
Diesel engines often achieve greater fuel efficiency than petrol (gasoline) engines. Diesel engines have energy efficiency of 45% and petrol engines of 30% [4]. That is one of the reasons why diesels have better fuel efficiency that equivalent petrol cars. A common margin is 40% more miles per gallon for an efficient turbodiesel. For example, the current model Skoda Octavia, using Volkswagen engines, has a combined European fuel efficiency of 38.2 mpg for the 102 bhp petrol engine and 53.3 mpg for the 105 bhp — and heavier — diesel engine. The higher compression ratio is helpful in raising efficiency, but diesel fuel also contains approximately 10-20% more energy per unit volume than gasoline.[5]
[edit] Fuel efficiency in microgravity
The energy output derived from fuel occurs during combustion. Ensuring a total, even combustion of fuel, as well as harnessable combustion at the appropriate moments, will have an impact on fuel efficiency. Recent research by the National Aeronautics and Space Administration (NASA) has gained possible insights to increasing fuel efficiency if fuel consumption takes place in microgravity. This probably does not apply to vehicles so much as industry where the benefit from the increased fuel efficiency will outweigh the initial cost of operating in a microgravity environment.
The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, such as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is that the cause is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs. [3] Experiments by NASA in microgravity reveal that diffusion flames in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions. [4] Premixed flames in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer. [5]
[edit] Fuel efficiency in transportation
- Humans (see Human-powered transport):
- walking or running one kilometre requires approximately 70 kcal or 330 kJ of food energy [6]. This equates to about 1 L/100 km or 235 mpg in gasoline energy terms.
- cycling requires about 120 kJ/km[6] This equates to about 0.36 L/100 km or 653 mpg in gasoline
- Since for each kcal of food energy, it requires several times as much fossil fuel energy to grow and transport the food, the efficiencies reported above need to be reduced accordingly.
- Airplanes: passenger airplanes averaged 4.8 L/100 km per passenger (1.4 MJ/passenger-km) (49 passenger-miles per gallon) in 1998. Efficiencies around 3 L/100 km per passenger are reached by some carriers. [7]. Note that on average 20% of seats are left unoccupied.
- Ships: the RMS Queen Elizabeth 2 gets 49.5 feet per gallon [8] (25,000 L/100 km or 13 L/100 km per passenger (3.8 MJ/passenger-km)). Note that about 40% of the power produced by the ship engines is used for propulsion, the rest being used to generate electricity for heating, lighting, and other passenger comforts.
- Trains:
- Freight: the AAR claims an energy efficiency of over 400 short ton-miles per gallon of diesel fuel in 2004[9] (0.588 L/100 km per tonne or 235 J/(km·kg))
- Passengers: the East Japan Railway Company claims for 2004 an energy intensity of 20.6 MJ/car-km, or about 0.35 MJ/passenger-km[10]
- Note that intercity rail in the U.S. reports 3.17 MJ/passenger-km which is several times higher than reported from Japan. Independent transportation researcher David Lawyer attributes this difference to the fact that the losses in electricity generation may not have been taken into account for Japan[11] and that Japanese trains have a larger number of passenger per car. [12]
- It should be noted that modern electric trains, like the shinkansen use regenerative braking to return current into the catenary while they brake. This method results in significant energy savings, where-as diesel locomotives (in use on unelectrified railway networks) typically dispose of the energy generated by dynamic braking as heat into the ambient air.
- This Swiss Railroad company SBB-CFF-FFS cites 0.082 kWh per passenger-km for traction, which is equivalent to 279 MPG [13]
- AEA carried out a detailed study of road and rail for the United Kingdom Department for Transport. Final report
- Rockets:
- The NASA space shuttle consumes 1,000,000 kg of solid fuel and 2,000,000 litres of liquid fuel over 8.5 minutes to take the 100,000 kg vehicle (including the 25,000 kg payload) to an altitude of 111 km and an orbital velocity of 30,000 km/h. Due to orbital decay this could not be a one-way trip, however if one supposes it is one-way for the purpose of illustration, this would amount to about 3,300 GJ of energy, or about 100,000 L/100 km or 12 feet per gallon (0.0023 mpg) of gasoline. In reality the shuttle continues to use its velocity to maintain a gradually-decaying orbit (analogous to driving to the top of a very tall mountain and coasting back down a very gentle slope, thus adding many miles to the distance travelled). The space shuttle Atlantis flew approximately 4.9 million miles on the STS-115 mission and this greater distance means much higher fuel efficiency for that particular mission. It's worth noting that a rocket can, in theory, re-entry on any place on Earth, giving it a best-case "ground" distance of 20,000 km. This would amount to 500 L/100 km or about 0.5 mpg.
- Transport comparison:
- The UK DfT state the following figures for public transport in 2005 [14]:
| Transport mode | Load factor (passengers per vehicle) | Fuel consumption (Miles per gallon per passenger) |
|---|---|---|
| Buses (national) | 9 | 98 |
| Passenger rail (diesel) | 90 | 182 |
| Air short haul | 100 | 40 |
| Air long haul | 300 | 66 |
- The US Transportation Energy book states the following figures for Passenger transportation in 2003 [15]:
| Transportation mode | Load factor (passengers per vehicle) | Fuel consumption (BTUs per passenger mile) |
|---|---|---|
| Cars | 1.57 | 3,549 |
| Personal Trucks | 1.72 | 4,008 |
| Motorcycles | 1.22 | 2,049 |
| Buses (Transit) | 8.7 | 4,160 |
| Air | N/A | 3,587 |
| Rail (Intercity Amtrak) | 17.2 | 2,935 |
| Rail (Transit Light & Heavy) | 21.7 | 3,228 |
| Rail (Commuter) | 33.4 | 2,571 |
Note: The actual MPG figure for each mode depends on the percentage of seats filled per vehicle, or load factor. Commuter rail and bus generally have lower load factors than air because of their 'walk on' nature. Whereas long distance rail and airlines use yield management techniques which raise loads, typically 71% for TGV services in France and 80-90% for Airlines. The overall load factor on UK railways is 33% or 90 people per train [16]:
- The US Transportation Energy book states the following figures for Freight transportation in 2003 [17]: [18]: [19]:
| Transportation mode | Fuel consumption (BTUs per ton mile) |
|---|---|
| Heavy Trucks | 3,357 |
| Class 1 Railroads | 344 |
| Air freight | 9,600 (aprox) |
| Domestic Waterbourne | 417 |
[edit] Transportation pollution
It’s important to realise that fuel efficiency doesn’t directly relate to emissions causing pollution and potentially leading to climate change. Rather, it depends on the fuel source used to drive the vehicle concerned. Cars can, for example, run on a number of fuel types other than gasoline, such as natural gas LPG or biofuel or electricity which creates various quantities of atmospheric pollution.
In the future hydrogen cars may be commercially available. Powered by chemical reactions in a fuel cell, that creates electricity to drive very efficient electrical motors; these vehicles promise to have zero pollution from the tailpipe. Potentially the atmospheric pollution could be near zero, provided the hydrogen is made by sustainable methods using solar, wind power or low CO2 energy sources such as nuclear power plants or hydro electric dams.
Currently railways can be powered using electricity, delivered to trains through an additional running rail or overhead catenary system. The atmospheric pollution, like electric cars, is no longer ‘at site’, rather at a distant power station. Some railways, such as SNCF and Swiss federal railways, derive most, if not 100% of their current from hydro or nuclear power stations, therefore atmospheric pollution from their rail networks is very low. [13]
Controversially, it’s thought by scientists that where emissions take place in the Earth’s atmosphere has an overall effect on climate change. Atmospheric changes from aircraft result from three types of processes: direct emission of radiatively active substances (e.g., CO2 or water vapor); emission of chemical species that produce or destroy radiatively active substances (e.g., NOx, which modifies O3 concentration); and emission of substances that trigger the generation of aerosol particles or lead to changes in natural clouds (e.g., contrails). What this means is that the total warming effect of aircraft emissions is 2.7 times as great as the effect of the carbon dioxide alone. [20]:
[edit] See also
- Annual fuel utilization efficiency (AFUE)
- Emission standard
- Energy conservation
- Fuel economy in automobiles
- Life cycle assessment
Electric motors are used to drive vehicles because they can be finely controlled, they deliver power efficiently and they are mechanically very simple. Electric motors often achieve 90% conversion efficiency over the full range of speeds and power output and can be precisely controlled. Electric motors can provide high torque while an EV is stopped, unlike internal combustion engines, and do not need gears to match power curves. This removes the need for gearboxes and torque converters. Electric motors also have the ability to convert movement energy back into electricity, through regenerative braking. This can be used to reduce the wear on brake systems and reduce the total energy requirement of a trip.
[edit] References
- ^ Appendix B, Trans. Energy Data Book
- ^ Automotive Handbook, 4th Edition, Robert Bosch GmbH, 1996. ISBN 0-8376-0333-1
- ^ CFM-1 experiment results, National Aeronautics and Space Administration, April 2005.
- ^ LSP-1 experiment results, National Aeronautics and Space Administration, April 2005.
- ^ SOFBAL-2 experiment results, National Aeronautics and Space Administration, April 2005.
- ^ a b [1]
- ^ IATA - Fuel efficiency, IATA
- ^ [2], Cunard Line
- ^ Railroads: Building a Cleaner Environment, Association of American Railroads
- ^ Environmental Goals and Results, JR-East Sustainability Report 2005
- ^ Fuel Efficiency of Travel in the 20th Century, Appendix
- ^ Fuel Efficiency of Travel in the 20th Century
- ^ a b SBB Environmental Report 2002/2003
- ^ Hansard, Commons Answers
- ^ Transportation Energy Data Book, 2006
- ^ [3] ATOC
- ^ Transportation Energy Data Book, 2006
- ^ US Environmental protection, 2006
- ^ EIA
- ^ Aviation and the Global Atmosphere, IPCC
